The invention relates to a master cylinder with a U-shaped seal. It is an object of the invention to improve the efficiency with which hydraulic braking systems fitted with such a master cylinder can be bled. The invention is more specifically intended for the field of motor vehicles but could be applied to other fields.
Master cylinders comprising a master cylinder body, a piston, a reservoir for brake liquid, or more generally for hydraulic fluid, and a pressure chamber, are known. The piston slides in a bore formed in the body of the master cylinder. The pressure chamber is connected to the hydraulic fluid reservoir via a hydraulic fluid pipe.
The master cylinder has a seal that allows the pressure in the pressure chamber to be increased. This seal is situated between a wall delimited by the bore of the body of the master cylinder and the piston. This seal prevents hydraulic fluid from passing from the pressure chamber to the hydraulic fluid reservoir. The passage of hydraulic fluid is prevented according to the position of the piston in the bore of the master cylinder. This seal is housed in a peripheral groove.
The peripheral groove is hollowed into the wall of the body of the master cylinder. This groove is circular cylindrical and faces the piston.
The seal is made of rubber and has a U-shaped profile. The profile comprises a base, the bottom of the U, to which two lips, the limbs of the U are connected. These two lips extend both in the same direction from this base.
A first lip, situated on the inside, is intended to face the piston. This lip extends from the smallest diameter of the base of the seal. This inner lip presses against the piston.
A second lip, situated on the outside, is intended to face the wall of the body of the master cylinder. More specifically, this second lip is intended to face the wall of the peripheral groove on the opposite side of the seal to the piston. This wall is formed by the closed end of the groove. The second lip extends from the largest diameter of the base of the seal. The outer lip rests against the closed end of the groove.
At rest, that is to say when a brake pedal of the vehicle is not being actuated, the pressure chamber is full of hydraulic fluid from the reservoir. The reservoir and the pressure chamber therefore communicate via a hydraulic fluid passageway.
When the piston moves forward in the bore of the body of the master cylinder, that is to say when the brake pedal is actuated, hydraulic fluid is prevented from flowing from the pressure chamber into the reservoir. What happens is that as it advances along the wall of the body of the master cylinder, the piston becomes positioned relative to the seal in such a way that the seal prevents hydraulic fluid from passing from the pressure chamber to the reservoir. The hydraulic fluid in the pressure chamber is therefore isolated from the hydraulic fluid contained in the reservoir. The forward movement of the piston then causes the pressure in the pressure chamber to rise. It then follows that the hydraulic fluid is injected from the pressure chamber into the vehicle braking system.
When the hydraulic braking system is being bled, the braking circuit, the outlet from the master cylinder is open. The expelled hydraulic fluid is collected in an open container positioned at a suitable location. To make this bleeding operation easier, the brake pedal is pumped hard. This pumping of the pedal forces the piston to move back and forth from its rest position to its forward position. This pumping of the pedal has the effect of discharging the liquid that is to be bled, together with any air bubbles that might be present in the hydraulic braking system.
As the piston moves along the wall of the master cylinder, during brake bleedings, a depression is created in the peripheral groove in which the seal is positioned. This depression is created as a result of the sharp forward movements of the piston in the bore. This depression will cause the lips of the seal to detach from the walls against which they respectively rest. This detachment may, in some cases, cause the lips to move closer to one another and cause the lips to move closer toward the wall facing an edge face of these lips. The inner lip and the outer lip will stick together. The two limbs of the U come into contact with one another at their ends. A vacuum will then be created between the lips of the seal in the thus-closed U. This vacuum forces the lips to remain stuck together as the piston returns to the rest position. This vacuum, is irreversible because, when the depression in the groove disappears, at best, the returning hydraulic fluid forces the two lips to remain stuck together. Once deformed in this way, the seal no longer seals.
Furthermore, a vacuum may also be created between the lips and the wall against which the edge face of the lips is stuck. This vacuum also forces the lips to remain stuck to the wall of the groove facing the edge face of the lips as the piston returns to the rest position, and once again the seal fails to perform its sealing function. The hydraulic fluid contained in the pressure chamber can then seep between the wall of the master cylinder body and the piston, as far as the hydraulic fluid reservoir.
The only way to avoid this lack of sealing is to bleed the brakes at a limited throughput and with a limited rate of pedal travel, otherwise the seal will no longer prevent hydraulic fluid from passing from the pressure chamber to the hydraulic fluid reservoir. It will be appreciated that bleeding in this way becomes a far more tricky operation. The operator has to have appropriate training and this training is an irksome constraint.
In an attempt to prevent these lips from sticking together it might be possible to increase the rigidity of the lips. To do that, an internal skeleton could be incorporated into the lips. Seals such as this with an internal skeleton could be produced using a dual molding technique. However, that would lead to complicated manufacture and a not-insignificant increase in cost.
Another solution might be to create passageways that cause the outside of the lips to communicate with the inside of the lips. These passageways would make it possible to prevent a vacuum from forming between the lips and between the lips and the wall when the pedal is being pumped. However, this solution would reduce the sealing of the lips and the seal would therefore no longer seal satisfactorily. In addition, producing such passageways in a seal of this size would have a tendency to weaken the structure of the seal. These solutions are therefore not acceptable for solving the problem such as it stands.
In order to solve this problem, in the invention, reliefs are made to prevent a vacuum from being created between the two lips. Thus, as the lips move closer together, the lips cannot stick tightly together because the reliefs prevent the lips from making a perfect seal with one another or with the wall facing the edge face of the lips. The fact that a perfect seal is not made between these lips and/or wall prevents the vacuum from being created. The vacuum that could create a depression between the two lips is removed via a communicating space allowing the outside and the inside of the lips to communicate with one another, this space being formed by virtue of the reliefs.
The invention of course can apply to a master cylinder that has several incorporated seals of this type. In addition, the invention can be applied to all kinds of master cylinder, including tandem master cylinders.
One subject of the invention is therefore a brake master cylinder comprising a piston sliding in a chamber of a master cylinder body, the body comprising an inlet allowing hydraulic fluid to enter the chamber, and a smooth seal of circular shape with a U-shaped profile, the U-shaped profile being formed by an inner lip and an outer lip of the seal, the seal being in contact via the inner lip with the periphery of the piston, and via the outer lip with the body, characterized in that it comprises localized reliefs produced on one lip.